A lifeless moon of a gas giant (which orbits a K-type star [4200K] at about 0.7 AU) is chosen to become home to a space colony. Colonists plan to transform it from a barren rock into a garden of Eden. Since this moon lacks water, nitrogen, and $CO_2$, they need to be mined in the asteroid belt and other gas giant moons and then delivered to the terraforming site. What is the most effective way to deliver these materials?

The colonists are looking for a delivery method that would provide results (such as atmosphere and free-flowing water) within years or decades. If it is absolutely impossible they can go into suspended animation and wake up in shifts to monitor the progress.

It would also be nice to avoid:

  • changes in orbit or rotation of the moon;
  • significant damage to the moon's surface;
  • creation of a debris cloud around the moon (the colonists are strongly opposed to space littering);
  • loss of already delivered materials (as seen in case of comet or asteroid bombardment).

Technological level

The colonists have access to the following technologies:

  • fully automated and robotised asteroid mining;
  • space travel at 1/10 of the speed of light;
  • terraforming technologies (however, only one project has been completed successfully by the time of their departure);
  • genetic engineering;
  • suspended animation.

Technologies that are envisioned by scientists of today but cannot be built because of technical difficulties (materials, money, political will) are fine. However, something like teleportation is not possible unless it can be explained by existing science.

  • $\begingroup$ By the looks of your conditions you're opposed to direct ballistic delivery $\endgroup$
    – Separatrix
    Commented Dec 26, 2017 at 20:03
  • $\begingroup$ @Separatrix, if ballistic delivery is indeed the only feasible way to do it, I will live with it. But other methods will be preferrable. $\endgroup$
    – Olga
    Commented Dec 26, 2017 at 20:06
  • $\begingroup$ One important read when considering terraforming projects is the wiki on approaches to terraforming Venus. It's gives good insight into various problems with such an undertaking. en.m.wikipedia.org/wiki/Terraforming_of_Venus $\endgroup$
    – Stephan
    Commented Dec 26, 2017 at 20:19
  • 2
    $\begingroup$ since most of the material you want to deliver directly to the atmosphere bombardment really has an advantage, it vaporizes the material at the same time it delivers it. of course drops do not need to be random you can use the bombardment to sculpt the surface to suit your needs. $\endgroup$
    – John
    Commented Dec 26, 2017 at 21:15
  • 1
    $\begingroup$ I was thinking with ridiculously convoluted gravity assists but the time frame doesn't permit that. How is can travel at 1/10 of the speed of light not the answer? Also, your "terraforming technologies" had better include knowing how to start a dead planet's dynamo. In the given time frame, that'd be more of a handwave then the engine. $\endgroup$
    – Mazura
    Commented Dec 26, 2017 at 23:41

3 Answers 3


Provenance of terraforming materials.

You mention getting things from the asteroid belt. There may be an easier way. Since you can move cargo fast (0.1c) you can afford to get the cargo from farther away. Asteroids are generally rocky with possibly some ices on top. Moons of Saturn are generally icy with some rocks in the middle.

There are a lot of moons of Saturn (and the moons of Uranus and Neptune are probably good targets as well). Collectively, they have far more ammonia and water than you could ever use to terraform a planet. So why not simply drag a few small to mediums sized moons of a gas giant into orbit around your planet and prepare to send them down?


Something not mentioned is the need to refine the materials. If you want the proper elements to be added to your moon in a matter of decades, then you have to be careful about what you add. Like any good culinary creation, you must measure your ingredients carefully.

You ingredients are bits of and/or whole moons. So how do you measure them? You have to melt them. You can utilize fractional distillation to melt away the various compounds. If you slowly cook (heat) a comet, all the Carbon monoxide will melt first (68 K), then methane (~91 K), ammonia (195 K), carbon dioxide (217 K) and finally water (273 K). All those temperatures are pretty far away from each other, so simply melt the ice ball slowly, and then separate the solid bits from the liquid at each step.

Now you have a set of liquid or slushy balls in space. If you were smart, you would do this far from the sun, so the carbon monoxide and methane will refreeze for you before transport. You now have a bunch of ice balls of reasonably pure compounds ready to go smash into your planet!

Recipe List

In the comments you say you want a plant with about 0.75 Earth's radius and mass; and 0.7 Earth's gravity. That doesn't work exactly, but going with some numbers that fit the bill more or less, let us assume your moon has radius 0.9 Earth's, density 0.8 Earth's, to get surface gravity 0.72 of Earth's. Mass ends up being 0.58 of Earth's.

Since surface area is proportional to radius, squared, we will need about 80% of the Earth's atmosphere, oceans, and biological matter. An atmosphere will need 20% oxygen and 80% inert gas; nitrogen is the most common inert gas and should do nicely. The requirements for our moon will be $3.3\times10^{18}$ kg of nitrogen and $2.1\times10^{17}$ kg of oxygen. The ocean will need $1.1\times0^{21}$ kg of water (though this could vary widely, depending on how wet you want the planet). Lastly, the biosphere will need at least $1\times10^{12}$ kg of carbon.

To provide these ingredients, we can add three compounds primarily. Ammonia can be used to generate atmospheric nitrogen; Carbon dioxide can be transformed in to atmospheric oxygen; and water is just water. At the ratio of two Ammonia per one diatomic nitrogen and one carbon dioxide per diatomic oxygen, our shopping list is roughly:

  • $1\times10^{21}$ kg water
  • $4\times10^{18}$ kg ammonia
  • $2\times10^{17}$ kg carbon dioxide

The great thing about these ingredients is that they are three of the most common compounds in the outer solar system. They also provide plenty of surplus material for making a biosphere: Carbon Dioxide has extra carbon and ammonia has extra hydrogen. No need to add methane, there are plenty of fossil fuels to go around!

How to not make a mess

The next challenge is to not make too big of a mess when you deliver your materials. Here are the various factors you outlined.

How not to significantly damage the moon's surface

Without an atmosphere, your moon will likely have a surface covered in fine regolith similar to what covers Luna and Mars. If this surface is hit by impacts from space, the dust will end up mostly settled back into the surface. So from this perspective, there isn't too much to damage done by hitting the planet with space snowballs; the holes will be filled by dust (relatively) soon after impact. Lunar regolith has a density about 2/3 of lunar surface rocks (and Earth rocks), so the holes will be filled with a material that will be reasonably solid.

Newton's depth approximation for impacts is $$D\approx L\frac{\rho_i}{\rho_p}$$ where L is the length (or diameter, if spherical) of the projectile and $\rho_i$ and $\rho_p$ are the densities of the impactor and planet, respectively. Note that this approximation nowhere includes the velocity of the impactor. Let us assume that planet has a similar crust density to Earth (2500 kg/m$^3$), while the delivered volatiles, such as CO$_2$, water, and ammonia each have densities less than 1000 kg/m$^3$.

Assuming we want to limit impact depth to 200 m so we don't make craters too large, we can throw objects up to 500m in diameter at the surface without making too much of a mess.

How not to not make a debris cloud

Putting stuff back into space will both anger your space-junk-OCD Chief Engineer and represent a loss of materials. We don't want to do that. How can we avoid it?

First, we have to figure out escape velocity of our planet. From this post, we see that radius and density are both proportional to surface gravity. As calculated above, we have radius 0.9 Earth's, density 0.8 Earth's, to get surface gravity 0.72 of Earth's. Mass ends up being 0.58 of Earth's.

Escape velocity is calculated here as $\sqrt{2gr}$, where $g$ is gravity and $r$ is radius. Given the factors above, escape velocity of your moon is 0.8 of Earth's, or 9000 m/s. To ensure nothing goes into space, we will make the average ejecta velocity from our impact craters no more than 4000 m/s.

In this post, I perform calculations on the height of an ejecta plume. This model calculates ejection velocity as a function of distance from the impact site. We want the ejecta velocity at the edge of the impactor to be less than 4000 m/s. If you work out the equation, you find that the maximum ejecta speed is proportional only to the impact velocity, and not to the mass or radius of the impactor (although the density of the impactor is very important). Ultimately, the relationship is $$4000 \text{ m/s} = .1313v_i.$$ Thus, for a 4000 m/s ejecta, the maximum impact speed must be about 30 km/s.

How not to eject volatile gasses into space

Of the gasses you are interested in, the two lightest and therefore most likely to escape are water (molar mass 18) and ammonia (molar mass 17). Therefore, we must figure out how to keep those gasses on the planet upon impact.

First, lets look at the ejecta plume from the last problem. Using basic kinematics, a particle (of ammonia) ejected at 4000 m/s, will reach a height of about 1100 km (Don't worry! I know that this is well into space, but without orbital velocity, it is coming back down!). The time it takes to get all the way up there is about 400 seconds, and the escape velocity at this height is about 8200 m/s.

Using the calculations in the answers to this post, we can figure out how hot an ammonia particle must be at this height to escape the moon's gravity. A particle must reach about 40000 K to escape under these conditions. Ouch! Now, individual particles are able to escape because the molecular distribution of kinetic energy has some variance to it. However, given that the escape velocity at the top of the ejecta blast is still about the same as the last linked post's calculated necessary escape velocity to hold gasses over geological time (8500 m/s at Earth's distance from the Sun), I think we can assume that very little of our gaseous ejecta will hit space.

How not to change the orbit and rotation of a moon

I had some more in depth calculations here, but they are not really needed. As long as you have the technology to accelerate things to 0.1c, I assume you have sufficient space horsepower to aim your delivered payloads as you like. If that is the case, then you simply hit the moon from all directions, so the net force of the impacts is zero.


Find a suitable mid-size moon. Melt it. Separate the various compounds into chunks of no more than 250m radius. Throw them into your planet at impact speeds of less than 30 km/s. Very little will escape into space. Profit!

  • $\begingroup$ The Chief Engineer is very pleased with your attention to his desire to keep space nice and clean. He is also very impressed with your suggestions. He wonders if strategic melting of frozen materials can be accomplished with mirrors? $\endgroup$
    – Olga
    Commented Dec 27, 2017 at 1:18
  • 1
    $\begingroup$ @Olga Mirrors would likely work too. I suggested solar panel powered lasers since they give you fine control over wavelength and heat delivered. You don't want to melt your moon all willy-nilly, you probably need to take years to heat it evenly and drive off the volatiles in just the right way so you can recover them. $\endgroup$
    – kingledion
    Commented Dec 27, 2017 at 1:27


I would think that mining of the asteroid belt, either manned or automated, could be done to break up the chunks into smaller than SUV-sized pieces that could be launched at the moon. This would avoid any littering of rocket or other man-made materials.

It would leave craters, but sizes this small wouldn't be as devastating as a full comet or asteroid. With it impacting the surface, it could help disbursement some, as well as creating friction heat to help bring up the temp of a barren planetoid.

Larger pieces could be used to make divots large enough to be a lake or reservoir, without the heavy machinery current methods require. Smaller pieces will avoid large blow back out of the intended atmosphere. Heavily pounding the rocky surface will actually help pulverize it into more easily planted soil.

There will likely need to be significant changes to the moons surface for humans to live there, so why not do it with the pot-shots of delivering material before we move in? Running water will change the surface, as will plants and the new weather patterns.

Also, adding mass in the form of air, water, etc. will change the orbit of the moon, so that is unavoidable, to a certain extent. We have changed the orbit of the Earth by creating lakes with dams and other water reservoirs.

An advantage of orbital bombardment is that it helps judge the level of available atmosphere. As the atmosphere forms, more and more friction will be shown on the debris. Once it gets near Earth density, most of the sub-SUV sized debris will never even hit the surface. This friction has the advantage of further disbursing the O2, N2, H2O, and other materials/minerals you are likely to need on the surface and in the atmosphere.


Using robotic miners would be faster than manned mining, but there could be a mixture of both, since the robots are likely to need maintenance. There's always the need for people to feed their families, so there's likely the "adventure seeker" that's willing to spend their time earning hazard pay for asteroid mining. After all, robots are expensive (they keep breaking) and humans are comparatively cheap (since there aren't enough jobs on Earth).

There's no need to render the materials to a refined state, just into small enough chunks. There could be a need to prevent certain volatile materials/substances from getting to the moon, but with the vast volume you are looking to fill, small pockets of even chlorine gas aren't likely to matter. And if you ship it with some sodium, it might even help, as in making salt.


There's the high likelihood of needing to use some sort of genetic modification of the micro and macro biological elements of the first stage of plants. The plants would need to be adapted to that exact environment. Not all plants can deal with the rocky, low CO2, low O2, low temp, low gravity, low moisture area you are talking about. These would likely need to also be high yield plants and microbes that would output high levels of O2, N2, and lots of other things to be able to create an atmosphere in even 100 years. This flora would also need to be able to break into the rocky surface to get the required minerals they need.

They may also need to be highly susceptible to a specific chemical or spray that would kill the fast spreading biome, so more Earth-like tame plants could be brought in, without fear of being killed off by the original planet evolving life forms.

Travel speed

Even 1/10th of the speed of light is really fast. This would allow us to go from the Earth to the asteroid belt in hours or days, rather than the current months, so a manned expedition is well within range for this speed. We would just need to make sure that we don't send any material into the moon at that speed.

You could, however, have a large transport that collects from the miners, then shoots over to the moon, slows down to open it's doors to offload/bombard the planet with the small fragments, then returns to the collection point. With 1/10th c, this could potentially be done in many points in the asteroid belt with nearly constant delivery to the moon.

The Martian Way, by Isaac Asimov, did something slightly similar. It is about a Mars colony that was having a shortage of water, asking the Earth to supply it. An Earth politician advocated against giving them more, citing a shortage of supply, so the Mars colony found their own solution. They sent out a group in a large rocket to find a large, mostly ice, asteroid to bring back. They ended up embedding the rocket into the asteroid and using it as a source of fuel to get home. They ended up with more than enough water for themselves, but had to expend a sizable portion of it to get it there and land it, rather than just crash it.



Large scale projects like this need to consider the economics of moving all that material around the Solar System. You will need to apply energy to move it from whatever orbits it is currently in, then, since you are opposed to ballistic impact, more energy to match the orbital speed of the target and deliver it at minimal speeds.

Depending on where the materials are in relation to the target, you have several choices. If you are in a farther orbit from the material source than the local sun, you can use high performance solar sails to tow the materials into the appropriate orbits. The sail can accelerate to the target planet, then "tack" by turning the trust vector against the direction of travel to match the orbital speed.

enter image description here

Solar sail accelerating to the target

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Solar sail decelerating to the target

While the usual image of solar sails is vast, slow moving devices, high performance sails with accelerations of 1mm/sec^2 can move across the Solar System at impressive speeds, a one way trip from Earth to Pluto at these speeds would only take 3 years (although that is a flypast). The real key is to set up a "pipeline" and send materials in a steady stream. While it may take 3 years for the first "package" to arrive, once the pipeline is filled, there is a steady stream of materials on the way.

enter image description here

K Eric Drexler pioneered the idea of thin film solar sails as far back as the 1970's

Using systems of mirrors at the target to reflect sunlight onto fast moving solar sails to assist slowing them down solves two issues, not only do you have finer control of incoming sails, but you can also use the solar energy when not controlling sails to provide energy to the surface, to assist in liquifying solids or turning liquid materials into gasses (an extreme case would be to focus solar energy onto the surface of Mars and boil Oxygen from the iron oxide on the surface. This is obviously energy intensive and inefficient, but with sufficient energy you can do almost anything).

Looking the other way, you could set up continental sized mirrors or platoons of mirrors to accelerate solar sails from the far reaches of the Solar System to send cut up pieces of comets back to the inner Solar System for your terraforming project. Given the weaker sunlight and vast distances, you might be looking at a decade before the first deliveries from the "pipeline" arrive, but once again, once the pipeline is filled, you have a steady stream of deliveries.

Without knowing important issues like the actual distances between the supply sources and targets, orbital velocities and so on, the answer is hand waved, but the ever useful Atomic Rockets site has a lot of relevant information and equations to work with so you can calculate delivery times, velocity changes etc.

  • $\begingroup$ I am still calculating distances, orbits, and velocities. So, unfortunately, I am unable to provide more detailed information at this time. However, your answer is incredibly helpful. $\endgroup$
    – Olga
    Commented Dec 27, 2017 at 1:26

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